1. Technical Field to which the Invention Belongs
The present invention relates to a technique capable of irradiating a large area with a laser beam having high uniformity, and also the invention relates to an application method thereof.
2. Prior Art
In recent years, extensive studies have been made on techniques in which laser annealing is performed on a non-single crystal semiconductor film (an amorphous semiconductor film which is not a single crystal, or a semiconductor film having crystallinity such as a polycrystalline and microcrystalline, and a semiconductor film in which these cristallinities are mixed) formed on an insulating substrate such as glass, to crystalize the film or to improve its crystallinity. A silicon film is often used for the above semiconductor film.
As compared with a quartz substrate that has been conventionally frequently used, the glass substrate has such advantages that it is inexpensive, it is superior in workability, and a large substrate can be easily formed. This is the reason why the above-mentioned researches are carried out. Further, the reason why a laser is preferably used for crystallization resides in that the melting point of the glass substrate is low. The laser is capable of giving high energy to only the semiconductor film without varying the temperature of the substrate very much.
Since a crystalline silicon film formed by performing a laser annealing to a silicon film has high mobility, it is used in such a manner that thin film transistors (TFTs) are formed with this crystalline silicon film, and are employed for, for example, a monolithic liquid crystal electrooptical device in which a TFT for driving pixels and a TFT for driver circuits are formed on one glass substrate. Since the crystalline silicon film is made of a large number of crystal grains, it is also called a polycrystal silicon film or a polycrystal semiconductor film.
A method in which a pulse laser beam of an excimer laser or the like having high output is processed by an optical system so that a square spot of several cm or a linear shape of several hundred xcexcmxc3x97several tens cm is formed on a surface to be irradiated, and the laser beam is made to scan (irradiation position of the laser beam is relatively moved to the irradiated surface) to make laser annealing, is superior in mass productivity and excellent in industry, so that the method is used by preference.
Particularly, when a linear laser beam is used, differently from the case of using a spot-like laser beam which requires back-and-forth and right-and-left scanning, laser irradiation to the whole irradiated surface can be made by scanning in only the direction normal to the line direction of the linear laser. Thus, high mass productivity can be obtained. The reason why scanning is made in the direction normal to the line direction is that it is the most effective scanning direction. By this high mass productivity, at present, laser annealing using the linear laser beam has become the mainstream.
When laser annealing is performed to the non-crystal semiconductor film by scanning of the laser beam that has been processed into a linear shape, rectangular, or square, some problems have occurred. One of especially serious problems among them is that the processing of laser beam is not uniformly carried out. When such a linear laser beam is used, laser annealing is nonuniformly performed onto the whole surface of the substrate.
FIG. 1 is a photograph of an optical microscope showing a state that a laser beam that has been processed into a linear shape by using a conventional optical system, is irradiated by one shot onto an amorphous silicon film. It can be observed irradiation marks at the center of the photograph.
FIG. 1 shows a case that an XeCl excimer laser having a wavelength of 308 nm is processed into a linear laser beam extending in the right-and-left direction on the paper surface, and is irradiated by one shot onto the amorphous silicon film.
It can be confirmed from FIG. 1 that edges in the width direction of the linear laser beam, particularly, an edge on the lower side of the paper surface has indentation, thereby being a linear laser beam having an uneven energy distribution.
FIG. 2a is a view schematically showing a state where a laser beam 201 having the uneven energy distribution shown in FIG. 1 is irradiated on a film 209.
As shown in FIG. 2a region 202 having a high energy density is formed at a center of the width direction, and regions 203 having a lower energy density compared to the region 202 are formed at peripheral portions of the width direction. FIGS. 2b and 2c each show sectional shapes of the energy distribution taken along the line X-Xxe2x80x2 and Y-Yxe2x80x2 of FIG. 2a. 
From FIGS. 2a to 2c, it can been seen that a laser beam 201 has different sectional shapes of the energy distribution in the width direction.
Laser annealing is performed onto the film by using the laser beam 201 of FIG. 2a, however, uniform laser annealing of the film can not be carried out.
The invention disclosed in the present specification has an object thereof to uniform the energy distribution in one direction of the laser beam, thereby uniformly performing the laser annealing of the film. Throughout the specification, xe2x80x9claser beamxe2x80x9d indicates a region of 5% or more of the maximum energy within the laser beam.
In general, in the case where a laser beam is processed into a linear shape, an originally substantially rectangular beam is made to pass through a suitable optical system and is processed into the linear shape. Although the aspect ratio of the substantially rectangular beam is about 2 to 5, for example, by an optical system shown in FIG. 3, it is transformed into the linear beam having an aspect ratio of 100 or more. At that time, the optical system is designed such that the distribution of energy in the beam is also homogenized at the same time.
The apparatus shown in FIG. 3 has a function to irradiate a laser beam, as a linear beam, from a laser beam generating unit 301 (in this state, the shape of the beam is substantially rectangular) through optical systems represented by 302, 303, 304, 306, and 308. Note that reference numeral 305 denotes a slit, and 307 denotes a mirror.
Reference numeral 302 denotes an optical lens serving to divide a laser beam in one direction, a linear direction in this case, and a cylindrical lens group (also referred to as a multicylindrical lens) is used. The divided many beams are overlapped and homogenized with respect to the linear direction by the cylindrical lens 306.
This structure is required to improve the strength distribution in the laser beam. The cylindrical lens group 303 also divides the laser beam in another direction, the width direction in this case, like the foregoing cylindrical lens group 302, and the divided beams are overlapped and homogenized with respect to the width direction by the cylindrical lenses 304 and 308.
That is, the combination of the cylindrical lens group 302 and the cylindrical lens 306 has a function to improve the strength distribution in the line direction of the linear laser beam, and the combination of the cylindrical lens group 303 and the cylindrical lenses 304 and 308 has a function to improve the strength distribution in the width direction of the linear laser beam.
In this case, with respect to the width direction, two cylindrical lenses 304 and 308 are used to make finer in the width direction of the linear laser beam on the irradiated surface 309. According to the width of the linear laser beam, the number of optical systems for overlapping may be made one, or may be made three or more.
The optical system serving to homogenize the energy distribution in the laser beam is referred to as a beam homogenizer. The optical system shown in FIG. 3 is also one of beam homogenizers. After the substantially originally rectangular laser beam is divided by the cylindrical lens groups 302 and 303, the divided beams each are shaped and overlapped by the cylindrical lenses 306, 304 and 308 to homogenize the energy distribution thereof.
Theoretically speaking, if the energy distribution of the laser beam is made even using a cylindrical lens group that includes infinite numbers of cylindrical lenses, a uniform laser beam can be obtained no matter what sectional shape the entered laser beam has.
However, in an industrially applicable cylindrical lense group, several, several tens at most, cylindrical lenses are used in consideration of its precision, cost, etc. In the cylindrical lens group as such, a laser beam is processed into a laser beam having an irregular energy distribution due to a sectional shape of the entered laser beam and the condition of entrance.
The present inventors have found that this unevenness, which has conventionally been considered as not much a trouble, causes a lot of problems mentioned above when that laser is used in laser annealing on a film represented by a thin film transistor (TFT) where minute elements are formed on the same substrate in a large number.
FIGS. 4a and 4b shows an example of, in a beam homogenizer for processing a laser into a linear laser beam, a cylindrical lens group 403 for dividing a laser beam in the width direction and a laser beam 401 entered thereto.
As shown in FIGS. 4a and 4b, laser beams 401, 401xe2x80x2 emitted from the beam generating unit 301 in FIG. 3 and entered into the optical system for dividing the laser beams have a substantially rectangular shape in its cross section.
Laser beams to be emitted from the beam generating unit 301 are ideally emitted with a perfect rectangular shape. However, it is realistically impossible with technologies present, and the sectional shapes thereof become substantially rectangular shapes.
In FIG. 4a, the laser beam is not entered over the entire width of the cylindrical lense in a cylindrical lens 4031 on the top and a cylindrical lens 4036 on the bottom. In addition, the entered beam shape is irregular.
On the other hand, the laser beam is entered over the entire width of each lens in four cylindrical lenses 4032 to 4035 placed therebetween.
FIG. 5 is a structural diagram showing an optical system for processing with respect to its width direction, taken out from the optical system for processing linear laser beams. As shown in FIG. 5, when a laser beam 501 is to be entered into a cylindrical lens group 503 in a manner shown in FIG. 4A, the laser beam enters to cylindrical lenses 5031 and 5036 with its edge irregular, not straight.
For that reason, the laser beams divided by the cylindrical lenses 5031 and 5036 are overlapped with each other on an irradiated surface 509 while the laser beams maintain the irregular shape by cylindrical lens 504. Thus formed is a linear beam that is not homogenous in the linear direction, namely a beam having an energy distribution whose sectional shape in the width direction varies In accordance with linear direction.
Then it becomes, for example, a linear laser beam in which a region 202 having a high energy density as shown in FIG. 2a is formed near the center in the width direction, and regions 203 having a lower energy density as compared to the region 202 are formed on the periphery in the width direction.
When the laser beam 401xe2x80x2 is to be entered to a cylindrical lens group 403xe2x80x2 for dividing the laser beam in the width direction as shown in FIG. 4b, a laser beam having an irregular shape enters to a cylindrical lens 4035,xe2x80x2 resulting in, similarly, a linear laser beam that is not homogenous in linear direction.
From the facts above, the present inventors have found out the followings. The cause of non-uniform laser annealing by a laser beam lies in an irregularly shaped laser beam that enters to some cylindrical lenses of a cylindrical lens group for dividing the laser beam. Because of this, the energy distribution of the linear laser beam becomes irregular.
According to one aspect of the present invention, there is provided a laser irradiation apparatus comprising: a lens for dividing a laser beam in one direction; and an optical system for overlapping the divided laser beams, characterized in that a shape of a laser beam entering into the lens has edges vertical to the direction.
Further, according to another aspect of the present invention, there is provided a laser irradiation apparatus comprising: a beam generating unit; a lens for dividing a laser beam in one direction; and an optical system for overlapping the divided laser beams, characterized in that a slit is formed between the beam generating unit and the lens, for forming edges in the laser beam, which is vertical to the direction.
Still further, according to another aspect of the present Invention, there is provided a laser irradiation apparatus comprising: an optical system for dividing a laser beam in one direction; and an optical system for overlapping the divided laser beams, characterized in that, in the direction, a width of the optical system for dividing is narrower than the maximum width of the laser beam before being divided.
Yet further, according to another aspect of the present invention, there is provided a laser irradiation apparatus comprising: a cylindrical lens group for dividing a laser beam in one direction; and an optical system for overlapping the divided laser beams, characterized in that a portion of the cylindrical lens of the cylindrical lens group is shielded.